1. Field of the Invention
This invention relates to a process for preparing by suspension polymerization a vinyl chloride polymer which has a particulate structure with a high bulk density and a high porosity, a sharp particle size distribution, and improved processability and is thus suitable for extrusion molding, injection molding and blow molding.
2. Prior Art
In the recent years, severer requirements are imposed on the quality of vinyl chloride polymers having a low degree of polymerization. Those vinyl chloride polymers intended for use in extrusion molding, injection molding and blow molding are required to have a particulate structure with a high bulk density and a high porosity and a sharp particle size distribution, and be easily processable.
For the manufacture of vinyl chloride polymers, a suspension polymerization method is well known in the art. This method is by suspending and dispersing vinyl chloride monomer in an aqueous medium for polymerization. In particular, vinyl chloride polymers having a high bulk density are manufactured by feeding additional vinyl chloride monomer during polymerization as disclosed in Japanese Patent Application Kokai (JP-A) No. 168008/1984, or using highly saponified polyvinyl alcohol as a dispersant for suspending and dispersing vinyl chloride monomer in an aqueous medium as disclosed in JP-A 7600/1982.
Also various attempts were made for producing vinyl chloride polymers which are easily processable and quickly gelled. For example, JP-A 5866/1977, 15890/1977, 6392/1978, and 112210/1980, and Japanese Patent Publication (JP-B) No. 18562/1986 disclose the preparation of vinyl chloride polymers using as a dispersant an oil-soluble polyvinyl alcohol having a degree of saponification of less than 55% and an average degree of polymerization of less than 600.
The vinyl chloride polymers obtained by these methods, however, have the problem that those polymers having a high bulk density have a slow gelling rate and fail to give acceptable moldings whereas those polymers having a high gelling rate and processability cannot be increased in extrusion molding rate. The prior art methods were difficult to produce a vinyl chloride polymer having a good balance of bulk density and gelling rate.